vi: Fix panic on UTF-8 char boundary in narrow terminal

Use char-based truncation instead of byte slicing when displaying
lines and messages. Removes redundant truncation in editor.rs since
expand_line already caps at max_cols.

Fixes #536

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: jgarzik

editors/tests/pty/mod.rs

@@ -225,3 +225,50 @@ fn test_pty_vi_delete_and_save() {
     assert_eq!(lines[0], "Line1");
     assert_eq!(lines[1], "Line3");
 }
+
+/// Test: UTF-8 display with multibyte characters in narrow terminal.
+/// Regression test for issue #536 - vi panics on UTF-8 char boundary.
+#[test]
+fn test_pty_vi_utf8_display() {
+    let td = tempdir().unwrap();
+    let file_path = td.path().join("test_utf8.txt");
+    // Cyrillic text "Привет мир" = "Hello world" - each Cyrillic char is 2 bytes
+    std::fs::write(&file_path, "Привет мир\n").unwrap();
+
+    let pty_system = native_pty_system();
+    let pair = pty_system
+        .openpty(PtySize {
+            rows: 10,
+            cols: 20, // Narrow terminal to force truncation
+            pixel_width: 0,
+            pixel_height: 0,
+        })
+        .unwrap();
+
+    let mut cmd = CommandBuilder::new(env!("CARGO_BIN_EXE_vi"));
+    cmd.arg(&file_path);
+    cmd.env("TERM", "vt100");
+
+    let mut child = pair.slave.spawn_command(cmd).unwrap();
+    drop(pair.slave);
+
+    let reader = pair.master.try_clone_reader().unwrap();
+    let _reader_thread = spawn_reader_drain(reader);
+    let mut writer = pair.master.take_writer().unwrap();
+
+    // Wait for vi startup
+    thread::sleep(Duration::from_millis(500));
+
+    // Move cursor right a few times (exercises display with UTF-8)
+    write_keys(&mut writer, "lll");
+    thread::sleep(Duration::from_millis(100));
+
+    // Quit without saving
+    write_keys(&mut writer, ":q!\r");
+
+    wait_with_timeout(&mut child, Duration::from_secs(5));
+
+    // If we got here without panic, the test passed
+    let contents = std::fs::read_to_string(&file_path).unwrap();
+    assert_eq!(contents, "Привет мир\n");
+}

editors/vi/editor.rs

@@ -3032,14 +3032,9 @@ impl Editor {
 
             if line_num <= self.buffer.line_count() {
                 if let Some(line) = self.buffer.line(line_num) {
-                    // Expand tabs and truncate
+                    // Expand tabs and truncate (expand_line already caps at max_cols)
                     let content = self.screen.expand_line(line.content(), size.cols as usize);
-                    let display = if content.len() > size.cols as usize {
-                        &content[..size.cols as usize]
-                    } else {
-                        &content
-                    };
-                    self.terminal.write_str(display)?;
+                    self.terminal.write_str(&content)?;
                 }
             } else {
                 self.terminal.write_str("~")?;

editors/vi/ui/screen.rs

@@ -1,5 +1,6 @@
 //! Screen buffer for efficient display updates.
 
+use super::display::truncate_to_width;
 use super::terminal::{Terminal, TerminalSize};
 use crate::buffer::Buffer;
 use crate::error::Result;
@@ -103,6 +104,11 @@ impl Screen {
         }
     }
 
+    /// Get tab stop width.
+    pub fn tabstop(&self) -> usize {
+        self.tabstop
+    }
+
     /// Mark all rows as dirty.
     pub fn mark_all_dirty(&mut self) {
         for row in &mut self.rows {
@@ -229,13 +235,10 @@ impl Screen {
         output.push_str(&format!("\x1b[{};1H", status_row));
         output.push_str("\x1b[K");
         if !self.message.is_empty() {
-            // Truncate message to fit
+            // Truncate message to fit (using UTF-8 safe truncation)
             let max_len = self.size.cols as usize;
-            if self.message.len() > max_len {
-                output.push_str(&self.message[..max_len]);
-            } else {
-                output.push_str(&self.message);
-            }
+            let msg = truncate_to_width(&self.message, max_len, self.tabstop);
+            output.push_str(&msg);
         }
 
         // Position cursor

pax/options.rs

@@ -540,8 +540,7 @@ pub fn format_list_entry(format: &str, info: &ListEntryInfo) -> String {
         if c == '%' {
             match chars.next() {
                 Some(spec) => {
-                    if let Some((_, handler)) =
-                        FORMAT_SPECIFIERS.iter().find(|(ch, _)| *ch == spec)
+                    if let Some((_, handler)) = FORMAT_SPECIFIERS.iter().find(|(ch, _)| *ch == spec)
                     {
                         result.push_str(&handler(info));
                     } else {